SUMMARY: The efficiency of oxidative phosphorylation in symbiotic nitrogen-fixing bacteria of the genus in the root nodules of leguminous plants is of fundamental importance as it influences the efficiency of ATP generation from the oxidation of plant photosynthate for the ATP-dependent process of nitrogen fixation. A poor efficiency of energy coupling was found for grown glucose-limited in chemostat culture as indicated by low growth yields on glucose [ = 80.3 g dry wt bacteria (mol glucose) and O [ = 25.6 g dry wt bacteria (mol O). (These values are low when compared with those for aerobic heterotrophic bacteria which have two or three sites of oxidative phosphorylation.) Although H/O measurements suggested a potential P/O ratio of 3 there was a very rapid rate of proton decay across the cell membrane; this coupled with the low growth yields suggested that the actual P/O ratio was nearer 1. Under conditions of excess dissolved oxygen cytochromes of the and types were detected. Under oxygen limitation a type terminal cytochrome oxidase was also produced and was associated with a further decrease in the efficiency of energy conservation. It remains to be seen whether such a low efficiency of energy conservation is expressed by within the root nodules of a host legume.


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